Ground fault receptacle

ABSTRACT

An electric receptacle for trade size outlet boxes having a ground fault circuit interrupting mechanism incorporated therein. A receptacle housing includes contact jaws seated therein to receive the plug of a conductor leading to a load, electronic components to detect a ground fault and grounded neutral condition on the load side of the receptacle, plus a coil and trip mechanism to open the receptacle circuit on occurrence of a ground fault or grounded neutral condition. The complete receptacle assembly is approximately the same depth as conventional household receptacles which do not have ground fault circuit interrupting means included therein. Means to periodically test the ground fault interrupting mechanism is also included in the receptacle assembly, as well as means to indicate the circuit has been interrupted and means to reset.

This is a continuation of application Ser. No. 674,714, filed Apr. 8,1976 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to the field of ground fault protective devicessensitive enough to protect humans from the hazards of electric shockresulting from ground fault currents. In the typical alternating currentinstallation, the neutral conductor is grounded at its source. Ifelectrical contact is made with the electrified or "hot" conductor by aperson standing on the ground, part of the current is diverted from itsnormal circuit through the electrified and neutral conductor and followsa ground path circuit through the person back to the neutral grounded atthe source. To protect against such hazard, ground fault interruptingdevices have been provided for installation in circuit breakers ofbranch circuits, and in portable units for temporary field use onconstruction sites. Attempts have also been made to mount such devicesin receptacles for installing in ordinary household outlet boxes whereground fault protection is most needed, such as bathrooms, kitchens andgarages. By having the entire ground fault sensing and interruptingmechanism in the receptacles where most needed, the entire branchcircuit is not interrupted every time the ground fault mechanisminterrupts the receptacle circuit.

A disadvantage of the receptacle mounted ground fault interruptingdevices hitherto known is their size, usually requiring oversize outletboxes. The ground fault receptacle in accordance with this invention iscompact enough to be installed in the same outlet boxes as conventionalreceptacles without ground fault protection which the receptacles inaccordance with this invention replace. It is not necessary to removethe original outlet boxes and install new ones of larger volume inbathrooms, kitchens and garages in order to obtain ground faultprotection wherever desired by using the new receptacles disclosed anddescribed herein.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a compact receptacle havingcomplete ground fault interrupting means which may be installed as areplacement for conventional receptacles in their original outlet boxes.

It is an object of the invention to provide a compact ground faultreceptacle having improved miniaturized components and circuits to makea more compact ground fault receptacle possible.

It is an object of the invention to provide a compact ground faultreceptacle having direct connection terminal means and beingsufficiently compact to permit direct connection of line-side andload-side conductors to said receptacle, eliminating the need forexternal pig tails for connection of said conductors to said receptacle.

It is an object of the invention to provide a ground fault receptaclehaving miniaturized components and circuitry, in which a rectificationbridge is provided having avalanche diodes with a reverse voltagebreakdown characteristic selected to prevent the D.C. voltage appearingacross said bridge from exceeding such pre-selected value, thuseliminating the need for a separate component to protect from nuisancetripping and damage due to high voltage conditions.

It is an object of the invention to provide a ground fault receptaclehaving miniaturized components and circuitry in which a ground faultsensing and interrupting circuit is powered on the line-side of thecircuit interrupting contacts, and in which electronic switch means forenergizing a trip coil to open said interrupting contacts is connectedto commutate off automatically after the ground fault has been cleared,thereby eliminating the need for a separate switch to open the line-sideconnected coil circuit.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a top plan view of a ground fault receptacle in accordancewith this invention.

FIG. 2 is a side elevation of the receptacle in FIG. 1.

FIG. 3 is a bottom plan view of the receptacle in FIG. 1.

FIG. 4 is a bottom plan view of a circuit board with electroniccomponents mounted thereon and terminals connected thereto.

FIG. 5 is a side elevation of FIG. 4 having a side terminal removed.

FIG. 6 is a section taken on line 6--6 of FIG. 4.

FIG. 7 is a section taken on line 7--7 of FIG. 4.

FIG. 8 is a bottom plan view of the movable contact carrier, reset andtrip assembly.

FIG. 9 is a section taken on line 9--9 of FIG. 8.

FIG. 10 is an end view of the contact carrier, reset and trip assembly.

FIG. 11 is a schematic drawing of the ground fault detection, groundedneutral detection and interrupting circuit in accordance with thisinvention.

FIG. 12 is a plan view of the receptacle case.

DESCRIPTION OF PREFERRED EMBODIMENT

A compact ground fault receptacle 1 in accordance with this inventionincludes an insulated receptacle case 2 and a receptacle cover 3. Ametal mounting strap or yoke 4 is secured to the case 2, and includesgrounding terminal 5 formed as an extension thereof.

The receptacle case 2 includes an intermediate compartment 6, aline-side terminal compartment 7 at one end of the intermediatecompartment 6 and a load-size terminal compartment 8 at the opposite endof intermediate compartment 6.

An open entrance-way 9 leads from intermediate compartment 6 toline-side terminal compartment 7. An open entrance-way 10 leads fromintermediate compartment 6 to load-side terminal compartment 8. Amounting pedestal 11 having a deep recess 12 therein opening to the top13 thereof is positioned at entrance-way 10 centrally thereof andinwardly of intermediate compartment 6. An insulating dividing wall 14extends from pedestal 11, at the midpoint thereof, into load-sideterminal compartment 8 to divide it into two compartment cells 15 and16. Vertical slots 17 and 18 are formed in exterior side walls 19 and20, opening to the upper edges 21 and 22 thereof. The dividing wall 14provides an insulating barrier to electrically insulate compartmentcells 15 and 16 from each other.

In line-side terminal compartment 7, vertical slots 23 and 24 are formedin exterior side walls 25 and 26 opening to top side edges 27 and 28.

A post 29 extends upwardly from the bottom 30 of intermediatecompartment 6 at the central region thereof for attaching communicationwith receptacle cover 3. Post 29 includes a central bore 31 opening tothe top, the bore 31 being internally threaded. When receptacle cover 3is in place over receptacle case 2, aperture 32 in cover 3 is inregistration with bore 31 of post 29. Anchor screw 33 is placed throughaperture 32 and rotated in base 31 for threaded engagement with post 29.

A printed circuit board 34 is dimensioned to overlay intermediatecompartment 6 and line-side terminal compartment 7, with the side edges34a and 34b of printed circuit board 34 extending flush with side walls6a and 6b of intermediate compartment 6 and resting on top thereof. Theforward ends of side edges 34a and 34b overhang the respective sidewalls 25 and 26 of line-side terminal compartment 7.

Line side terminals 35a and 36a are affixed to the forward end ofcircuit board 34, one on each side thereof, to depend downward instraddling relationship with the side walls 25 and 26 of line-sideterminal compartment 7. Terminal screws 37 and 38 are threadedly mountedthrough corresponding threaded apertures in the depending side walls ofterminals 35a and 36a respectively. The shanks 37a and 38a of saidterminal screws project inwardly from their respective terminal sidewalls with the respective inner ends of said terminal screws in opposedfacing relationship. When the printed circuit board 34 with terminals35a and 36a depending from the side edges at the forward end thereof isplaced over the top of intermediate compartment 6 and line-side terminalcompartment 7 for installing therein, the shanks 37a and 38a arereceived respectively in slots and in side walls 25 and 26 of theline-side terminal compartment 7.

The ground fault sensing and grounded neutral detecting components andthe tripping mechanism are mounted on the inward facing side 39 ofprinted circuit board 34. These include two differential transformersT-1 and T-2 comprising a ground fault sensing toroid 40 and a couplingtoroid 41 respectively, through which a "hot" or electrified conductorL1 and a neutral conductor N1 extend to constitute the primary windingsthereof. The electrified conductor L1 is connected to line-side terminal35a on one side of toroids 40 and 41, and after passing through saidtoroids, it is connected to load side terminal 35b. The neutralconductor N1 is connected to line-side terminal 36a on one side oftoroids 40 and 41, and after passing through said toroids, it isconnected to a load side terminal 36b.

The differential transformer T-1 which includes toroid 40 functions as aso-called zero sequence transformer to sense the occurrence of a groundfault on the load side of the electrified conductor L1. When no groundfault is present, the magnetic fields resulting from current flow inelectrified conductor L1 in one direction and in neutral conductor N1 inthe opposite direction are of opposite polarity and equal. The magneticfields thus cancel out. However, when a ground fault occurs in theelectrified conductor L1 on the load side of toroid 40, a portion of thecurrent returns to source through a ground path rather than throughneutral conductor N1. Thus, the respective magnetic fields ofelectrified conductor L1 and neutral conductor N1 are unbalanced as theypass through toroid 40 where they constitute the primary winding of thisdifferential transformer. Accordingly the magnetic fields do not cancelout, and a net amount of magnetic flux is available to be picked up insecondary winding 44 on toroid 40 and thus induce a voltage signaltherein.

The detection and interruption circuit is powered as follows. A fullwave bridge 45 having avalanche characteristics is connected across lineconductor L1 and neutral conductor N1 on the line or source side ofinterrupting contacts 103 and 104, by means of conductor 48 connected toone side of the bridge and conductor 49 connected to the other side andextending to terminal 50 of trip coil 51. Conductor 52 extends from tripcoil terminal 53 to neutral conductor N1. Bridge 45 provides a rectifiedpower supply to an integrated circuit component 54 through conductors 55and 56 connected to pins 57 and 58 respectively of I.C. component 54.

The I.C. component 54 includes therein an operational amplifier, voltageregulator and level detector. The pins designated 57 and 58 representthe voltage regulator portion, pins designated 59, 60, 61, 62 and 63represent the operational amplifier portion, and pin 64 represents thelevel detector portion.

As stated above, a rectified power supply is fed from bridge 45 to I.C.54 and connected to pins 57 and 58 which represent the voltage regulatorportion of the I.C. chip 54 and which sets the appropriate voltage levelfor the operational amplifier portion of the chip.

When a ground fault occurs in line conductor L1, on the load side of thetoroid 40, part of the current returns to source through a ground pathrather than through the neutral conductor N1, creating an imbalance inthe respective magnetic fields of L1 and N1 where they pass throughtoroid 40. As described above, a net amount of magnetic flux is thusavailable to induce a voltage signal in secondary winding 44. Thisvoltage signal is transmitted to the operational amplifier stage of I.C.54 by way of an input circuit comprising conductor 65 leading to pin 59(an inverting input terminal of the operational amplifier stage), andconductor 66 leading to pin 60 (a non-inverting input terminal of theoperational amplifier stage). Conductor 67 leads from terminal 68 ofsecondary winding 44 to the junction 69 with conductor 65. Conductor 70leads from the other terminal 71 of secondary 44 to the junction 72 withconductor 66.

Diodes 73 and 74 are connected in parallel with secondary winding 44 toprevent saturation of the transformer toroid core 40 during very highvalues of ground fault current.

When the induced voltage signal transmitted from secondary winding 44 isreceived on pins 59 and 60 of the op amp stage of I.C. 54, it istransmitted to the output pin 61, from which it flows through a negativefeed back path, comprising conductor 75 to junction 76 and then to aninverting input 62 through conductor 77 and a one megohm potentiometer78. This negative feedback path controls the gain of the amplifierstage, and the potentiometer may be adjusted to control the ground faulttrip current. For example, it may be adjusted so when there is 5milliamps difference between the currents in line conductor L1 andneutral conductor N1, the amplifier output peak voltage exceeds thereference voltage of the level detector stage, supplied by the voltageregulator stage which in turn receives a DC voltage supply on pins 57and 58 from bridge 45 through a voltage dropping resistor 79. When theoutput peak voltage of the amplifier stage exceeds the referencevoltage, a D.C. voltage is produced at pin 64 of the level detectorstage which triggers silicon controlled rectifier (SCR) 80 intoconduction.

Capacitor 81 is connected across input terminals 62 and 63 of the op ampstage of I.C. 54 to provide a low pass filter for additional immunity tospurious noise signals.

The D.C. voltage from pin 64 of I.C. 54 is fed to the gate of SCR80through conductor 82. Capacitor 83 is connected across the cathode-gatecircuit of SCR80 to prevent it from triggering and tripping the circuitdue to noise on the circuit which could be amplified by I.C. 54. A highenough noise level will trip the SCR80 and is preferred to as a firstpredtermined noise level.

When SCR80 is triggered into conduction line voltage is applied to tripcoil 51 causing it to trip thus opening contacts 46 and 47 to interruptthe power line circuit. When SCR80 conducts, a circuit is completed fromN1 through conductor 52, terminals 50 and 53 of trip coil 51, conductors49 and 84 to SCR80, conductors 85 and 56, zener diode 86 and conductor48 to L1.

The full wave rectifier bridge 45 includes zener diodes 86, 87, 88 and89. The diodes are selected to avalanche with a reverse voltage between200 and 300 volts peak. If a voltage transient, a predetermined secondlevel of noise voltage in excess of 300 volts peak occurs betweenconductors L1 and N1 of the power circuit, the avalanche or zener diodes86-89 will avalanche and clip the voltage at a safe amplitude to protectSCR80 and I.C. 54 from damage. The impedance of trip coil 51 acts as achoke to limit current sufficiently on occurrence of such hightransients to protect the diodes 86-89 from being damaged. By using arectification bridge of this type with avalanche or zener diodes, anadditional component such as a metal oxide varistor (MOV) used in otherground fault protector circuits is not needed for impulse protection.The avalanche rectification bridge in accordance with this inventionperforms the dual functions of rectification and protection from highvoltage transients.

In accordance with this invention, the ground fault protection circuitis powered from the line side of the interrupting contacts 46 and 47. Inthis way, the ground fault protection circuit remains powered even afterthe power circuit has been interrupted by opening of contacts 46 and 47.In other devices of this type which power the ground fault protectivecircuit from the line side, a separate switch is used to deenergize thetrip coil after tripping for a ground fault. In the present invention, aseparate switch is not needed for this purpose. The problem ofde-energizing the trip coil after tripping for a ground fault is solvedby connecting the anode end 90 of SCR80 to the A.C. side of bridge 45rather than to the D.C. side as is customary in prior art devices. Theanode end 90 is connected to the A.C. power line circuit throughconductors 84, 49, coil 51 and conductor 52 to the neutral conductor N1.This means that the SCR80 operates in the half wave mode and commutatesoff once every cycle. Therefore, when the fault is removed by opening ofthe contacts as the next line voltage excursion through zero, the SCR80turns off.

A test circuit is provided which includes a 15 kilohm resistor 91 inseries with test switch 92 in conductor 93 extending from junction 94with line conductor L1 on the load side of the differential transformertoroid 40 to junction 95 with conductor 49 leading to neutral conductorN1 on the line side of toroid 40. Thus, when test switch 92 is closed,current imbalance occurs at toroid 40 as a result of current bypassingthe neutral conductor at that point returning to neutral from junction94 through the test circuit. If the ground fault protection circuit isworking properly, such current imbalance will induce a pick-up signalwhich will be amplified and trip the circuit in the manner described.

The ground fault protection circuit also includes protection against aground on the neutral conductor which if not detected and cleared wouldadversely affect the sensitivity of the circuit. Protection against agrounded neutral is provided as follows.

A coupling transformer T-2 including toroid 41 having a winding 96 isconnected to the output 61 of the operational amplifier stage of I.C.54, by means of a feedback circuit. Conductor 97 extends from terminal98 of the winding 96 to junction 76 to receive an output from terminal61 of the op amp. A capacitor 99 and resistor 100 are connected inseries in conductor 97 to complete a regenerative feedback path from theoutput stage of the op amp to the transformer winding 96 on toroid 41 oftransformer T-2. The other terminal 101 of winding 96 is connectedthrough conductor 101a to terminal 71 of the secondary winding 44 ontoroid 40 of the differential transformer T-1.

Transformer T-2 and the circuit in which it is connected are quiescentwhen conditions in the power line circuit are normal and no ground ispresent on conductor N1 on the load side of transformer T-2. However, ifthe neutral wire N-1 is grounded on the load side of toroids 40 and 41through an impedance of 4 ohms or less, a feedback circuit then existsthrough the one turn loop created by neutral wire N-1 passing throughboth toroids 40 and 41, which thereby magnetically couples thetransformers T-1 and T-2. This feedback loop causes the operationalamplifier stage of the I.C. to oscillate. Such oscillation is detectedby the internal level detector stage of the I.C. in the same manner as asignal voltage resulting from occurrence of a ground fault. An outputvoltage thereupon appears on pin 5 of the level detector stage of theI.C., which gates the SCR 80 into conduction thereby causing trip coil51 to open contacts 46 and 47, thus interrupting the circuit.

The circuit and components described above, will therefore interrupt thepower line circuit both on occurrence of a ground fault on the load sideof the toroids 40 and 41 and on occurrence of a grounded neutral on theload side of toroids 40 and 41.

The circuit interruption and reset mechanism includes a two-pole breakerassembly having a movable contact carrier 102 mounted for movementbetween a contact open and contact closed position. Stationary contacts103a and 104a are mounted respectively on conductive end portions ofline-side terminals 35a and 36a, contact 103a on electrified or "hot"terminal 35a and contact 104a on neutral terminal 36a.

Movable contacts 103b and 104b are mounted on lateral planar projections105 and 106 of movable contact carrier 102 in alignment respectivelywith stationary contacts 103a and 104a for contact therewith whenmovable contact carrier 102 is moved to the contact closed position.

An integral projecting member 107 of contact carrier 102 extendsoutwardly from the planar surface thereof and between contacts 103b and104b mounted on each side of the carrier. The projecting member 107 hasa cross sectional configuration and dimension corresponding to that of aguide recess 108 in guide member 109 mounted on the inward facing sideof printed circuit board 34, for sliding movement of projecting member107 into and out of said guide recess 108, as the carrier 102 moves intocontact closing position and away from the stationary contacts intocontact open position.

Projecting member 107 includes a cylindrical recess 110 opening to side111 of movable contact carrier 102 which faces in the direction ofmovement toward the contact open position. At the opposite end ofcylindrical recess 110 is an axially aligned aperture 112 which opensinto guide recess 108 of guide member 109 when contact carrier 102 isoperably positioned for movement between contact closing and contactopening positions with projecting member 107 aligned with guide recess108.

Guide recess 108 also includes an axially aligned aperture 113 throughits roof portion 114.

An elongated pin 115 having a head 116 at one end with circumferentiallyprojecting edge 117 extending radially outward from shank 118, ismounted with shank 118 extending through the axially aligned apertures112 (of cylindrical recess 110 in the contact carrier projection 107)and 113 (of guide recess 108) with an end portion of shank 118projecting beyond the roof 114 of guide member 109 to fixedly receivereset button 119 thereon.

Reset button 119 includes an axially aligned seating recess 120 openingto receive the end of shank 118 of pin 115 for securing it to the bodyof reset button 119 by means of a lock plate 121 seated in an annulargroove at the end of shank 118. A cap 119a welded on reset button 119traps the lock plate 121 in place to secure the end of shank 118 toreset button 119.

A helical latching spring 122 is loosely mounted on and co-axial withthe upper portion of shank 118, between the outward facing side of domedroof portion 114 of guide member 109 and the inner end of seating recess120 of reset button 119. The latching spring 122 bears against said roofportion 114 at one end and against said inner end of seating recess 120at the other end, to bias said reset button in an unlatching directionaway from said roof portion 114. When reset button 119 is fully biasedaway from domed roof portion 114 of guide member 109, thecircumferentially projecting edge 117 of head 116 of elongated pin 115is in abutting engagement against the inner end 123 of cylindricalrecess 110 of the projecting member 107 of movable contact carrier 102.The projecting end 124 of projecting member 107 is at this time near theouter end 125 of guide recess 108.

A helical disconnect spring 126 is loosely mounted on and co-axial withthe lower portion of shank 118 of elongated pin 115, between theprojecting end 124 of projecting member 107 of contact carrier 102 andthe inner end of guide recess 108 of the guide member 109. Thedisconnect spring 126 bears against said projecting end 124 of member107 at one end and against said inner end of guide recess 108 at theother end, to bias said movable contact carrier and contacts 103b and104b in a disconnect direction away from said stationary contacts 103aand 104a.

A latch 127 is mounted on the forward wall 128 of projecting member 107of contact carrier 102, with latch finger 129 projecting resilientlyinto the cavity of cylindrical recess 110 near the open end 130 thereof.When reset button 119 is pushed inwardly fully against the bias ofhelical latch spring 122, the rounded surface 131 of head 116 of pin 115pushes the resiliently projecting latch finger 129 outwardly to permitthe head 116 of pin 115 to pass whereupon the latch finger 129 snapsback to project inwardly of cylindrical recess 110. Thecircumferentially projecting edge 117 of the pin head 116 thereuponlatches against latch finger 129, and latch spring 122 then biases resetbutton 119 when released in a direction outwardly from the domed roofportion 114 of guide member 109. This carries elongated pin 115 andmovable contact carrier 102 latched to the pin 115 to a contact closedposition, whereby movable contacts 103b and 104b are brought intocontact respectively with stationary contacts 103a and 104a.

The projecting member 107 of contact carrier 102 is at such time drawninto guide recess 108, which compresses disconnect spring 126 to providespring loading for rapid separation of movable contacts 103b and 104baway from stationary contacts 103a and 104a when the tripping mechanismcauses latch finger 129 to release head 116 of elongated pin 115.

The latch 127 includes a latch release plate portion 132 aligned withplunger type armature 133 which moves axially into unlatching engagementwith latch release plate 132 when trip coil 51 is energized. When latchrelease plate 132 is thus engaged by armature 133, it is moved by thearmature and carries latch finger 129 out of latching engagement withhead 116 of pin 115, thus releasing projecting member 107 of contactcarrier 102 and permitting it to move against the bias of disconnectspring 126 to the contact open position, thus interrupting the powerline circuit.

At such time, reset button 119 is moved outwardly under the bias oflatching spring 122 whereupon red band 134 around an intermediateportion of reset button 119 appears outwardly of outer surface 135 ofreceptacle cover 3 to indicate a tripped condition with the contactsseparated.

To reset and close the contacts, reset button 119 is depressed until thehead 116 of elongated pin 115 catches on latch finger 129. When button119 is released, latching spring 122 biases reset button 119 outwardlycarrying pin 115 and contact carrier 102 with it until the movablecontacts 103b and 104b are in contact with their respective stationarycontacts 103a and 104a. The receptacle circuit is now set and ready foruse when a load is connected to load side terminals 35b and 36b.

Jaw members 136 and 137 project from load-side terminals 35b and 36brespectively, and grounding jaw member 138 projects from a groundingconnection with mounting strap 4, to receive the corresponding prongs ofan electrical plug connected to a load.

Jaw members 136 and 137 seat in respective compartment cells 15 and 16of the receptacle case 2, and grounding jaw member 138 is seated inrecess 12 of mounting pedestal 11 of receptacle case 2.

Load side terminals 35b and 35b include terminal screws 139 and 140respectively, threadedly mounted through corresponding threadedapertures in the depending side walls of terminals 35b and 36brespectively. The shanks 139a and 140a of said terminal screws projectinwardly from their respective terminal side walls. When the terminals35b and 36b with their respective jaw members 136 and 137 are seatedwithin the receptacle case 2, the projecting shanks 139a and 140a of theterminal screws are received through vertical slots 17 and 18 of sidewalls 19 and 20 of load-side compartment 8 of receptacle case 2.

Printed circuit board 34 lies on top of case 2 when the line side andload side terminals are seated in their respective compartments 7 and 8as described. The electronic components mounted on inward facing side 39of circuit board 34 lie within the intermediate compartment 6. Resetbutton 119 projects upwardly from the outward facing side 141 of circuitboard 34, and test contact element 142 is also mounted on the outwardfacing side 141. End 142a of element 142 is connected to conductor 49leading to trip coil 51 and end 142b thereof is resiliently spaced fromcontact with conductor 93 leading to the load side of line L1 throughresistor 91.

Test button 143 includes recess 144 therein to receive contact element142 therein. Contact element 142 is resilient, so test button 143 may bedepressed when contact element 142 is seated therein to make contactwith conductor 93 for testing. When test button 143 is released, theresilience of contact element 142 carries end 142b out of contact withconductor 93 and moves test button 143 outwardly.

Receptacle cover 3 is placed over printed circuit board 34 to hold itsecurely in place on receptacle case 2 with the electronic componentsdepending from the inward facing side 39 thereof. Aperture 145 in cover3 receives reset button 119 therethrough and aperture 146 receives testbutton 143 therethrough for external operation. When cover 3 is in placeover the top of receptacle case 2, the internally threaded bore 31 ofpost 29 is in registration with aperture 32 in cover 3. Screw 33 securescover 3 to case 2 when fully threaded into bore 31 of post 29. Cover 3also include prong receiving apertures 147, 148 and 149 in registrationwith jaw members 136, 137 and 138, to receive the prongs of anelectrical plug for connection of the receptacle to a load.

The ground fault receptacle assembly in accordance with this inventionis compact enough to be mounted in outlet boxes of ordinary household ortrade size, including all of those boxes for devices listed in Table370-6(a) of the National Electrical Code which is reproduced below:

                                      TABLE 370-6(a).                             __________________________________________________________________________    Boxes                                                                                           Min.                                                        Box Dimension, Inches                                                                           Cu. In.                                                                            Maximum Number of Conductors                           Trade Size of Type                                                                              Cop. #14                                                                              #12                                                                              #10                                                                              #8 #6                                         __________________________________________________________________________    4 × 11/4 Round or Octagonal                                                               12.5 5  5  5  2  1                                          4 × 11/2 Round or Octagonal                                                               15.5 7  6  6  5  0                                          4 × 21/8 Round or Octagonal                                                               21.5 10 9  8  7  0                                          4 × 11/4 Square                                                                           18.0 9  8  7  6  0                                          4 × 11/2 Square                                                                           21.0 10 9  8  7  0                                          4 × 21/2 Square                                                                           30.3 15 13 12 10 0*                                         4 11/16 × 11/4 Square                                                                     25.5 12 11 10 1  0                                          4 11/16 × 11/2 Square                                                                     29.5 14 13 11 9  0                                          4 11/16 × 21/8 Square                                                                     42.0 21 18 16 11 6                                          3 × 2 × 11/2 Device                                                                 7.5  3  3  3  2  0                                          3 × 2 × 2 Device                                                                    10.0 5  4  4  3  0                                          3 × 2 × 21/4 Device                                                                 10.5 5  4  4  3  0                                          3 ×  2 × 21/2 Device                                                                12.5 6  5  5  4  0                                          3 × 2 × 21/4 Device                                                                 14.0 7  6  5  4  0                                          3 × 2 × 31/2 Device                                                                 18.0 9  8  7  6  0                                          4 × 21/8 × 11/2 Device                                                              10.3 5  4  4  3  0                                          4 × 21/8 × 17/8 Device                                                              13.0 6  5  5  4  0                                          4 × 21/8 × 21/4 Device                                                              14.5 7  6  5  4  0                                          33/4 × 2 × 21/2 Masonry Box/gang                                                    14.0 7  6  5  4  0                                          31/4 × 2 × 31/2 Masonry Box/gang                                                    21.0 10 9  8  1  0                                          FS--Minimum Internal Depth 13/4                                               Single Cover Gang 13.5 6  6  5  4  0                                          FD--Minimum Internal Depth 21/8                                               Single Cover Gang 18.0 9  8  7  6  1                                          FS--Minimum Internal Depth 13/4                                               Multiple Cover Gang                                                                             18.0 9  8  7  6  0                                          FD--Minimum Internal Depth 23/8                                               Multiple Cover Gang                                                                             24.0 12 10 9  8  0                                          __________________________________________________________________________     *Not to be used as a pull box. For termination only.                     

The words "trade size" when used in this specification and claims inrelation to outlet boxes refer to the box dimensions in the foregoingtable.

The outer dimensions of the ground fault receptacle assembly of thisinvention, including the receptacle case 2 and cover 3 are as follows:

Depth (a): From planar outer surface 150 of cover 3 to bottom planarsurface 151 of case 2, one and one-sixteenth inches.

Depth (b): From the inner faces 152 of mounting ears 153 to the bottomplanar surface 154 of mounting strap 152 and of bottom planar surface151 of case 2, both of which surfaces are coplanar, one inch.

Width: Across the widest portion, one and five-eighths inches.

Length: From co-planar end surfaces 155 and 156 of case 2 and cover 3 atone end to the opposite coplanar end surfaces 157 and 158, two andthree-fourths inches.

The ground fault receptacle assembly of this invention is accordinglycompact enough and small enough to be inter-changeable with conventionalhousehold receptacles not having internally contained ground faultinterrupting means, and it can be installed in the same outlet box assuch conventional household receptacle which it may be used to replace.

The terminal compartments 7 and 8 in accordance with this invention,have a narrower width than the intermediate compartment 6, whereby thereis room for screw type terminals and external access thereto, includingterminal screws 37 and 38 on the line side and terminal screws 139 and140 on the load side. Such compact structure of the receptacle andground fault interrupting mechanism encased therein, eliminates the needfor pig tail type terminals extending from the receptacle. The inventionas described herein provides a ground fault receptacle having screw typeterminals enabling connection of power line conductors directly to thebody of the receptacle.

The cover 3 includes two raised regions 159 and 160, each having aperipheral configuration and dimension corresponding to the receptaclereceiving apertures in standard household type duplex face plates.Apertures 145 and 146 are formed in raised region 159, and prongreceiving apertures 147, 148 and 149 are formed in raised region 160 ofthe receptacle cover 3.

The mounting strap 4 includes two mounting ears 152, each respectivelyprojecting outwardly in opposite directions from opposite ends of thereceptacle 1. Apertures 161 are provided in said mounting ears 152. Themounting ears 152 and apertures 161 are spaced apart and positioned tocorrespond with the spaced apart mounting apertures of trade size outletboxes for devices such as receptacles. The receptacle assembly inaccordance with this invention is thereby useable in all of the tradesize outlet boxes listed in Table 370-6(a) above which have suchmounting apertures for mounting devices therein.

We claim:
 1. A circuit for providing immunity from noise and nuisance tripping for a ground fault protective device having two or more electrical contacts connected to an electrical distribution system having two or more conductors, said circuit comprising:a means for tripping the electrical contacts of the ground fault protective device, said means for tripping being part of said device, and connected to a first of the two conductors; a power supply connected directly to a second of the two conductors and also connected to said means for tripping, said means for tripping being a selected impedance for limiting current to said power supply when noise voltage in the electrical distribution system exceeds a predetermined second level so that energy dissipated by said power supply does not exceed said power supply's dissipative capacity, said power supply having means for clipping the noise voltage when the noise voltage exceeds the predetermined second level; a ground fault responsive means for producing a fault signal, said ground fault responsive means being electrically connected to said two conductors; a switching means connected to said ground fault responsive means and responsive to the fault signal, said switching means being a silicon controlled rectifier having its anode connected to the AC side of said power supply and also to said means for tripping, its gate connected to circuitry in said ground fault responsive means, and its cathode-gate connected to the DC side of said power supply, thereby causing said silicon controlled rectifier to operate in the half wave mode and commutate off once every cycle so that, when a fault is removed by opening the electrical contacts, at the next line voltage excursion through zero, the silicon controlled rectifier turns off, said switching means being responsive to a predetermined first level of noise voltage, said switching means causing said means for tripping to be energized by said power supply when said switching means receives the fault signal from said ground fault responsive means; and wherein said predetermined second level of noise voltage is less than said predetermined first level of noise voltage thereby preventing said switching means from responding to noise voltage in the distribution system and, also, preventing inadvertent energizing of said means for tripping. 